Advances in ceramic processing have permitted the replacement of various components of electrical and mechanical equipment with sintered ceramic parts. Ceramics have found widespread use in electronic components, cutting tools, and to a lesser degree, as structural substitutes for metals. However, the properties exhibited by ceramic materials are determined by the sintered microstructure, and under current processing systems the properties can be highly variable, depending in large part upon the quality of the starting powder, and especially the size distribution of the particles.
Ceramic structures are typically manufactured by packing powders to form a so-called "green body" that is subsequently sintered at high temperature to yield the ceramic material. At present, commercially available powders have large size distributions, typically from less than 0.5 micrometers to about 10 micrometers. Because of such wide size distributions, orderly packing of powders into green bodies has been difficult, and relatively high sintering temperatures have been required. Consequently, sintered microstructures and properties such as shrinkage variability and surface finish have not been well controlled. Accordingly, there exists a need for a system of producing narrow size fraction powders.
The use of centrifugation for size classification (separating particulate matter into size fractions) is known in the art. See generally Perry, Chemical Engineers' Handbook.
Separating powders into narrow particle-size ranges is accomplished through sedimentation. Sedimentation rate is given by Stokes' law of settling (Ex. 1): ##EQU1## where v=a particle's settling velocity, h=the distance through which the particle settles, t=the time required for the particle to settle through distance h, r=the particle radius, g=acceleration due to gravity, p.sub.p =particle density, p.sub.m =density of the medium, .eta.=liquid viscosity, and K=the particle-shape factor (2/9 for a sphere), which takes into account both a particle's volume and its cross-sectional area.
The sum of a medium's buoyant force and the drag on a submicrometer particle makes simple gravitational settling time-consuming and tedious, and therefore uneconomical. Increasing the settling forces through centrifugal sedimentation speeds settling. Because a particle's terminal velocity is proportional to the square of its size, large particles settle through a medium considerably faster than do smaller particles, allowing easy separation. For centrifugal separation, the Svedberg-Nichols modification of Stokes' law is applicable (Eq. 2): ##EQU2## where t=the time required for a particle to settle through a distance x.sub.2 -x.sub.1, for x.sub.2 =the rotating radius of the centrifuge to the end point of the particle's travel path and x.sub.1 =the rotating radius of the centrifuge to the beginning point of the particle's travel path; w=angular velocity in radians/sec.
Under traditional approaches, a specific particle-size classification ("cut") is achieved by first calculating the angular velocity and residence time required to force particles larger than the largest desired size out of the dispersion to form a sediment on the wall. The dispersion is placed in a centrifuge bowl and then centrifuged under these calculated conditions, and the resulting overflow, containing only particles finer than the upper limit of the desired increment, is decanted. The overflow is then processed in a fashion similar to that used for the original dispersion, so that all particles larger than the lowest size desired are spun out of suspension onto the centrifuge wall. This second sediment consists of particles within the desired size range and is therefore retained.
This "batch" centrifugation of the prior art is not adapted for production of narrow size distribution powders on a commercial scale. The batch process has insufficient throughput, and does not produce an optimally narrow size distribution powder. This is in part because at the beginning of each centrifugation procedure, particle distribution in the dispersion is random, so as particles within the desired size range are forced out of suspension, smaller particles in their path are dragged along with them, into the sediment. There exists a need for a carefully controlled system for generating narrow size distribution powders. Such powders would find widespread use and satisfy a variety of long felt needs in forming high-performance ceramics. A further use for narrow size distribution powders is for fillers in conjunction with ferrite-ceramic compositions.